The present invention is an improved process for the reaction of an alkyl halide with particulate silicon in a fluidized-bed process. The improvement comprises controlling the particle size of the silicon within a range of one micron to 85 microns. Preferred is when the particle size of the silicon has a mass distribution characterized by a 10th percentile of 2.1 to 6 microns, a 50th percentile of 10 to 25 microns and a 90th percentile of 30 to 60 microns. Most preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5, a 50th percentile of 12 to 25 microns, and a 90th percentile of 35 to 45 microns. The process is run in the presence of a catalyst composition comprising copper and other catalysts.
The present invention relates to an improvement of what is commonly referred to as the Direct Process for producing alkylhalosilanes, where the process comprises contacting an alkyl halide with particulate silicon in the presence of a copper catalyst. The process was first described by Rochow in U.S. Pat. No. 2,380,996 and U.S. Pat. No. 2,380,995, issued Aug. 7, 1945.
Since the original description of the Direct Process by Rochow, the process has been refined and modified in numerous ways and is used for producing virtually all commercial alkylhalosilanes in the world today. When one considers that several million pounds of silanes are produced annually and consumed by the silicones commercial effort, it is obvious why even small incremental increases in selectivity and raw material conversion are important to the manufacturer of alkylhalosilanes.
Commercially, the largest volume alkylhalosilane manufactured is dimethyldichlorosilane as this alkylhalosilane constitutes the backbone of most high volume commercial silicone products after it has been hydrolyzed and condensed to form silicone polymers. Therefore, it is to the benefit of the manufacture to run the Direct Process to maximize the conversion of the raw materials to obtain the highest yield of dialkyldihalosilane. Thus one of the objectives of the present invention is to control the Direct Process to maximize the overall yield of dialkyldihalosilane, i.e. to cause the process to be as selective as possible in favor of dialkyldihalosilane. A second objective of the present invention is to maximize the overall yield from the raw materials. The more of the raw materials that are converted to silanes, the more economical is the process. A third objective is to provide a process where less silicon spent bed of reduced activity in the process is produced.
The art has long recognized that in the Direct Process the size of the particulate silicon is important in determining the efficiency of the reaction of alkyl halides with silicon to form alkylhalosilanes. For the purpose of the present invention the efficiency of the reaction of alkyl halides with silicon is tracked by the amount of silicon charge that is converted to dialkyldihalosilane. However, the prior art has generally taught away from the optimal particle size range discovered and now disclosed by the present inventors.
Reed et al., U.S. Pat. No. 2,389,931, issued Nov. 27, 1945, first taught the use of a fluidized-bed of particulate silicon to conduct the Direct Process. Reed et al. taught that the silicon reactant should be in finely-divided or powdered form. By way of example. Reed et al. described the use of a sintered silicon-copper mixture crushed to 60-100 mesh (149 to 250 microns).
Gilliam et al., U.S. Pat. No. 2,466,413, issued Apr. 5, 1949, appear to be the first to report the effect of silicon particle size on performance of the Direct Process. Gilliam et al. reported that an optimal particle size for silicon in the Direct Process was a distribution where 100% of the particles are smaller than 420 microns in diameter, from 90 to 100% of the particles less than 149 microns in diameter and not more than 60% of the particles are less than 44 microns in diameter. Gilliam et al. used a packed-bed reactor to arrive at this optimal particle size distribution.
Dotson, U.S. Pat. No. 3,133,109, issued May 12, 1964, describes a process reported to be an improvement on Reed et al., supra. The improvement of Dotson being (1) adding to the fluidized bed make-up silicon having an average particle size greater than the average particle size of the silicon comprising the fluidized bed and (2) comminuting at least intermittently in a non-oxidizing atmosphere at least a portion of the silicon being adjusted to maintain a substantially constant average particle size for the silicon comprising the fluidized bed. Dotson teaches that for optimal results the silicon in the reactor should have an average particle diameter in the range of from about 20 to 200 microns. Dotson teaches preferably at least 25 percent by weight of the silicon particles have actual diameters in the range of from 20 to 200 microns.
Maas et al., U.S. Pat. No. 4,218,387, issued Aug. 19, 1980, describe a vibrating-bed type reactor for conducting the Direct Process on a laboratory scale. The silicon evaluated was reported to have a particle size distribution characterized as follows: &lt;36 .mu.m (31.3%): 36-71 .mu.m: (22.6%): 71-100 .mu.m: (17.8%): 100-160 .mu.m: (18.0%): and &gt;160 .mu.m (10.3%).
Shade, U.S. Pat. No. 4,281,149, issued Jul. 28, 1981, describes a process for extending the activity of a fluidized bed used in the Direct Process. The process described by Shade consists of removing particles selected from particles of silicon and copper of less than 40 microns average diameter size from the reactor and abrading said particles to remove the surface coating of such particles and returning the particles to the reactor. Shade teaches that for optimal results the silicon in the reactor has an average particle diameter in the range of from about 20 to 200 microns. Preferably at least 25 percent by weight of the silicon particles have actual diameters in the range of from 20 to 200 microns.
Shah et al., U.S. Pat. No. 4,307,242, issued Dec. 22, 1981, also report a fluidized-bed Direct process where the silicon in the reactor has an average particle diameter in the range of from about 20 to 200 microns. Preferably at least 25 percent by weight of the silicon particles have actual diameters in the range of from 20 to 200 microns.
Ward et al., U.S. Pat. No. 4,554,370, issued Nov. 19, 1985, teach the use of fumed silica in a mixture of powdered silicon and cuprous chloride to reduce agglomeration in a fluidized-bed reactor during conduct of the Direct Process. Ward et al. teach that the silicon present in the fluidized bed can have a particle size below 700 microns, with an average particle size of greater than 20 microns and less than 300 microns in size. The mean diameter of the silicon particles is preferably in the range of 100 to 150 microns.
Ward et al., U.S. Pat. No. Re. 33,452, issued Nov. 20, 1990, teach the use of a copper-zinc-tin catalyst in the direct process. Ward et al. teach that significant improvements in reaction rate and product selectivity are achieved when copper is employed at a critical weight percent relative to silicon and critical weight ratios of tin and zinc are employed relative to copper. Ward et al. teach that the process can be conducted in a fluidized-bed using silicon having a particle size below 700 microns, with an average size of greater than 20 microns and less than 300 microns in size. The mean diameter of the silicon particles is preferably in the range of 100 to 150 microns.
In view of the cited art, quite unexpectly the present inventors have found that the optimal silicon particle size for conducting the Direct Process in a fluidized-bed reactor is within a range of about one micron to 85 micron. Preferred is when the particle size of the silicon has a mass distribution characterized by a 10th percentile of 2.1 to 6 microns, a 50th percentile of 10 to 25 microns, and a 90th percentile of 30 to 60 microns. Most preferred is when the particle size mass distribution of the silicon is characterized by a 10th percentile of 2.5 to 4.5, a 50th percentile of 12 to 25 microns, and a 90th percentile of 35 to 45 microns. The inventors have discovered that particles outside of the described ranges degrade the efficiency of the reactor and therefore it is preferable that these sizes of particles not be added to the reactor or if added be controlled within defined mass ranges.